1. Field of the Invention
The present invention relates to a steel for a welded structure superior in a CTOD property of a heat affected zone (HAZ) in a low heat input welding to a medium heat input welding, and a producing method thereof. Particularly, the present invention relates to a steel for a welded structure far superior in a CTOD property of an FL zone and an IC zone where toughness deteriorates the most in a low heat input welding to a medium heat input welding, and a producing method thereof.
This application is a national stage application of International Application No. PCT/JP2010/003435, filed May 21, 2010, which claims priority to Japanese Patent Application No. 2009-124614 filed on May 22, 2009, Japanese Patent Application No. 2009-123428 filed on May 21, 2009, and Japanese Patent Application No. 2009-192387 filed on Aug. 21, 2009, and the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, there has been a demand for a steel for use in harsh environments. For example, as high-strength steel suitable for steel structures such as offshore structures used in a frigid sea area such as the Arctic region, and seismic resistant structures, there is a need for a steel excellent in a CTOD (Crack Tip Opening Displacement) property which is one of fracture toughness parameters. In particular, the weld of the steel needs an excellent CTOD property.
The CTOD property of the heat affected zone (HAZ) is evaluated by test results of two positions (notch section) of an FL zone “Fusion Line: a boundary of a WM (weld metal) and an HAZ (heat affected zone)” and an IC zone “Intercritical HAZ: a boundary of an HAZ and a BM (base metal)”. However, only the FL zone considered to obtain the lowest CTOD property has been evaluated in the past.
In conditions where a test temperature is not particularly harsh, for example, −20° C., if the CTOD property of the FL zone is sufficient, the CTOD property of the IC zone is also sufficient, such that it is not necessary to evaluate the CTOD property of the IC zone.
However, under harsh test conditions, for example, −60° C., there are many cases where a CTOD value of the IC zone is not sufficient, such that it is necessary to increase the CTOD property of the IC zone.
In this respect, techniques in which the CTOD property at a harsh test temperature (for example, −60° C.) of welded joint after low heat input and medium heat input is improved are disclosed (for example, refer to Patent Citation 2). However, in these techniques, the CTOD property of the IC zone is not disclosed.
In addition, for example, techniques in which Charpy absorbed energy of welded joint after a large heat input welding is improved are disclosed (for example, refer to Patent Citation 3 to Patent Citation 6). However, in these techniques, the CTOD property (property of brittle crack initiation) that is an important factor as a structural material is not disclosed, and the test is only performed with respect to a relatively high temperature condition (for example, −10° C.).
In the above-described techniques, for example, a relatively large amount of O is contained in steel for securing a sufficient amount of Ti-oxides which act as transformation nuclei for the generation of an intragranular ferrite (IGF) in the FL zone. In addition, for example, for making a microstructure fine after welding, an element, which stabilizes austenite and increases hardenability, is added in a certain amount or more. However, in this method, it is difficult to secure the CTOD value of the IC zone of the steel in a harsh environment of about −60° C. while securing properties (for example, the strength or toughness of a base metal, and the CTOD value of the FL zone) necessary for a structural material for welded structure.
[Patent Citation 1] Japanese Unexamined Patent Application Publication, First Publication No. 2007-002271
[Patent Citation 2] Japanese Unexamined Patent Application Publication, First Publication No. 2008-169429
[Patent Citation 3] Japanese Unexamined Patent Application Publication, First Publication No. 2002-030380
[Patent Citation 4] Japanese Unexamined Patent Application Publication, First Publication No. 5-171341
[Patent Citation 5] Japanese Unexamined Patent Application Publication, First Publication No. 2004-162150
[Patent Citation 6] Japanese Unexamined Patent Application Publication, First Publication No. 11-279684